Semiconductor laser diodes have found applications in a wide variety of information handling systems because of their compact size and because their technology is compatible with that of the associated electronic circuitry. They are being employed in areas such as data communications, optical storage and optical printing. Most commonly used are group III/V compound materials. Particularly AlGaAs lasers have found extensive usage.
Historically, the mirror facets have been obtained by cleaving the laser bars, i.e., layered structures forming the active waveguide of the device. Cleaving usually provides single, often high quality devices which require, however, further individual processing and testing. More recently, there is a strong trend to increase the scale of integration which requires the replacement of at least one cleaved mirror facet of the laser diodes by an etched mirror. Since substantial progress has been made in obtaining good quality etched mirrors, this technology is very promising. It allows processes like mirror coating and testing to be performed on the wafer level--with the benefit of reduced handling, increased yield, and decreased fabrication and testing costs.
For both types of lasers, i.e., those with cleaved and those with etched mirrors, one of the most important and critical device criteria is the reliability and thus the maximum lifetime of the device at the output power levels required for the various applications. Reliability and device lifetime are heavily affected by mirror contamination and degradation which lead to device heating and finally to device destruction.
Over a long period of time, in fact for more than 15 years, proposals and attempts have been made trying to solve these problems. Most successful has been the coating of the mirror facets with passivation layers which are applied to protect the mirror surfaces against contamination during the lifetime of the device. For this standard passivation the power dissipated at the mirror turned out to be responsible for a gradual degradation close to the facet. This degradation may lead to a catastrophic optical mirror damage (COMD), as described in "Thermodynamics approach to catastrophic optical mirror damage of AlGaAs single quantum well lasers", A. Moser, E.-E. Latta, D. J. Webb, Applied Physical Letters, Vol. 55, No. 12, p. 1152, 1989.
The time dependent COMD event, observed at semiconductor laser diodes with standard passivation, can be completely suppressed by application of the in-situ passivation process described and claimed in the U.S. Pat. No. 5,063,173 with title "Method for Mirror Passivation of Semiconductor Laser Diodes", and the corresponding counterpart patents in various countries. For devices which were passivated in-situ, an absolute COMD level exists. The laser can be operated below this COMD level without any mirror damage, i.e., the laser devices which were passivated in-situ are very reliable. Lifetimes of up to 21/2 years have been recorded under tough conditions in a long distance fiber link where the semiconductor lasers are employed as pump lasers in pump modules.
The above mentioned absolute COMD level depends on the physical properties of the passivation layer used. For increasing absorption the absolute COMD level moves downward. In today's semiconductor lasers, damages and/or degradation at the laser facet where the light wave is decoupled from the standing light wave within the laser's cavity occur due to the relatively high intensity at the facet.
To the best of our knowledge, semiconductor lasers so far built or reported, still suffer from a number of deficiencies:
Continuous operation output powers of above 200 mW are still not possible for an extended period of time; PA1 There is still room for improvement of the device lifetimes.
It is a main object of the present invention to provide reliable semiconductor diode lasers and a method for the fabrication of reliable semiconductor diode lasers.
It is another object of the present invention to provide high-power, long lifetime, high performance semiconductor diode lasers and a method for the fabrication of such high-power, long lifetime, high performance semiconductor diode lasers.